The present invention relates to pressure rolls which can be used in calenders and like machines to apply pressure to the surfaces of running webs of paper, textile material or the like. More particularly, the invention relates to improvements in pressure rolls of the type wherein a hollow cylindrical shell is rotatable about a stationary carrier and is centered on such carrier by one or more rows of hydraulic supporting elements.
The end portions of the carrier for the shell of a pressure roll of the above outlined character are mounted in the frame of the machine. It is further known to mount on the carrier bearing units for the end portions of the shell and to provide each bearing unit with an inner bearing element which is installed in the respective end portion of the shell and is held against axial movement with reference to the carrier. Such inner bearing element can serve as a part of, or it may constitute, a thrust bearing and/or it can perform the function of a seal for the respective end portion of the shell. The hydraulic supporting elements ensure that the external surface of the shell bears against the external surface or surfaces of one or more adjoining pressure rolls, e.g., in a calender wherein a running web must be advanced through the nips of two or more cooperating rolls to enhance the quality of its surfaces and/or other characteristics. Analogous pressure rolls can be used with advantage in smoothing, grinding, embossing, printing, pressing or other types of machines for the treatment of webs of textile material, synthetic plastic material or paper as well as in rolling mills for steel or other metallic materials.
British Pat. No. 641,466 discloses a pressure roll wherein hydraulic plungers constitute supporting elements and their end faces abut against the internal surface of the shell. The plungers form a row intermediate the axis of the shell and the nip between such shell and the adjoining pressure roll. By changing the pressure in the cylinder chambers for the plungers, one can regulate the pressure between the shell and the adjoining pressure roll, and it is also possible to lower the shell away from contact with the adjacent pressure roll by permitting some or all of the hydraulic fluid to escape from the cylinder chambers. The bearing unit at each axial end of the shell comprises an outer bearing element which is secured to the respective end portion of the shell and an inner bearing element which is spacedly surrounded by the outer bearing element so that the two bearing elements provide room for insertion of an antifriction roller bearing therebetween. The bearing units which are disclosed in the aforementioned British patent are capable of taking up axial stresses, i.e., of transmitting axial stresses from the shell to the carrier which supports the hydraulic supporting elements and is surrounded by the shell. When the cylinder chambers for the plungers are disconnected from the source of pressurized hydraulic fluid and are connected to the sump, the inner bearing elements of the two bearing units serve as stops which limit the extent of downward movement of the shell, i.e., the inner bearing elements intercept the shell after the latter moves its external surface through a predetermined distance in a direction away from the adjacent pressure roll. In other words, the inner bearing elements of the two bearing units limit the extent of radial movement of the shell with reference to its carrier. The bearing units which are disclosed in the British patent are further equipped with means for sealing the respective ends of the shell, i.e., for preventing the penetration of impurities into the interior of the shell and/or for preventing escape of lubricant from the shell.
German Pat. No. 1,026,609 discloses a modified pressure roller wherein the radial movements of the shell with reference to its carrier can take place only in a so-called supporting plane, namely, in a plane which includes the axis of the shell and the axis of the adjoining pressure roll. This is achieved by providing the carrier with guide means which permit the shell to move radially of the carrier but confine the radial movement to that in the just mentioned supporting plane. The guide means comprise radially extending pins or studs whose axes are located in the supporting plane and which are anchored in the end portions of the shell to extend into complementary bores or holes of the carrier, or vice versa. Analogous guide means are disclosed in German Pat. No. 22 54 392 wherein the carrier has diametrically extending bores for the pins or pairs of movement-restricting surfaces which are parallel to the supporting plane. Reference may also be had to U.S. Pat. No. 3,885,283 granted May 27, 1975 to Biondetti.
A drawback of the aforedescribed conventional pressure rolls is that the guide means and/or the bearing units prevent highly accurate adjustments of the shell with reference to its carrier and/or with reference to the cooperating pressure roll as well as that the pressure between the external surface of the shell and the external surface of the adjoining pressure roll cannot be selected with a requisite degree of precision. Moreover, the pressure cannot be regulated with the same degree of accuracy all the way between the two axial ends of the shell.
A further pressure roll is disclosed in commonly owned U.S. Pat. No. 4,328,744, granted May 11, 1982 to Pav et al., which employs hydrostatic supporting elements each having a battery of cylinder and piston units disposed one next to the other, as considered in the circumferential direction of the shell. Each cylinder and piston unit can receive pressurized fluid independently of the other cylinder and piston units to thus ensure a much more accurate adjustment of pressure between the peripheral surface of the shell and the external surface of the adjacent pressure roll. The patent to Pav et al. further discloses that the shell can surround two rows of hydrostatic supporting elements, namely, a row above and a row below the carrier. This further increases the number of possible adjustments, for example, so as to compensate for the resultants of several forces acting upon the shell in two or more different directions. Such forces can include the force of gravity, the bias of the adjoining pressure roll and the forces which are applied by a running web of textile or other material advancing through the nip of the shell and the adjacent pressure roll.